> or it gets defeated by constant folding

thats the point, compare the optree of my (0?$a:$b)="X" with my ($b)="X".

Deparse has no chance to see any difference 

lanx:~$ perl -MO=Concise -e 'my (0?$a:$b)="X";print $a' 
c  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 1 -e:1) v:{ ->3
7     <2> aassign[t3] vKS ->8
-        <1> ex-list lK ->5
3           <0> pushmark s ->4
4           <$> const[PV "X"] s ->5
-        <1> ex-list lK ->7
5           <0> pushmark s ->6
6           <0> padsv[$b:1,2] lPRM*/LVINTRO ->7
8     <;> nextstate(main 2 -e:1) v:{ ->9
b     <@> print vK ->c
9        <0> pushmark s ->a
a        <0> padsv[$a:1,2] l ->b
-e syntax OK

[download]
lanx:~$ perl -MO=Concise -e 'my ($b)="X";print $a' 
c  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 1 -e:1) v:{ ->3
7     <2> aassign[t2] vKS ->8
-        <1> ex-list lK ->5
3           <0> pushmark s ->4
4           <$> const[PV "X"] s ->5
-        <1> ex-list lK ->7
5           <0> pushmark s ->6
6           <0> padsv[$b:1,2] lPRM*/LVINTRO ->7
8     <;> nextstate(main 2 -e:1) v:{ ->9
b     <@> print vK ->c
9        <0> pushmark s ->a
-        <1> ex-rv2sv sK/1 ->b
a           <#> gvsv[*a] s ->b
-e syntax OK

[download]

Note line a: In the first example $a is coded as entry "$a" from a lexical pad and in the second it's the scalar value of glob *a from a symbol table.

IMHO you found a case where the compiler should better warn about undefined behavior.

Cheers Rolf

UPDATE: we had a discussion about this just few weeks ago, but I can't find it. Take this instead: 'my' with 'if 0' retains old value

UPDATE: And here the warning I'm missing:

perl -wce 'my $a if 0; print $a' 
Deprecated use of my() in false conditional at -e line 1.
-e syntax OK

[download]

UPDATE: Found it! see A curious case of of my() and Re^4: A curious case of of my() (What is GLOBAL?)

In reply to Re^3: my (0?$a:$b): a koan by LanX
in thread my (0?$a:$b): a koan by educated_foo

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